Neuromonitoring is a subfield of clinical patient monitoring focused on measuring various aspects of brain function and on changes therein caused by neurological diseases, accidents, and drugs commonly used to induce and maintain anesthesia in an operation room or sedation in patients under critical or intensive care.
Electroencephalography (EEG) is a well-established method for assessing brain activity. When measurement electrodes are attached on the skin of the skull surface, the weak biopotential signals generated in brain cortex may be recorded and analyzed. The EEG has been in wide use for decades in basic research of the neural systems of the brain as well as in the clinical diagnosis of various central nervous system diseases and disorders.
While spontaneous variation in a wake-sleep cycle causes physiological and rapidly reversible changes in the EEG, different derangements of internal system homeostasis disturb the environment in which the brain operates, and therefore the function of the brain and the resulting EEG are disturbed. The EEG signal is a very sensitive measure of the neuronal derangements, which may reflect in the EEG signal either as changes in membrane potentials or as changes in synaptic transmission. A change in synaptic transmission occurs whenever there is an imbalance between consumption and supply of energy in the brain. This means that the EEG signal serves as an early warning of a developing injury in the brain.
Generally, if a patient is unconscious (without sedation), the reason in 30 to 40 percent of the cases is intracranial, whereas in 60 to 70 percent of the cases unconsciousness is due to hypoxic-ischaemic, metabolic, or toxic reasons. This kind of general unconsciousness is currently monitored with the help of the Glasgow Coma Scale (GCS). It defines the patient (un)consciousness by using three parameters: the best eye opening response, the best motoric response, and best response to speech. The final score represents the sum of the scores of the three categories. Although the Glasgow Coma Scale is subjective and inter-rater variability may exist, it is the most widely used scoring system to assess patients with traumatic brain injury, for example.
Diagnostically, the EEG is only rarely specific, since many systemic disorders of the brain produce similar EEG manifestations. However, an EEG signal may be of critical value, as it may differentiate between broad categories of psychogenic, epileptic, metabolic-toxic, encephalitic, and focal conditions, for example.
In a healthy sleeping subject, the EEG is reactive to various stimuli depending on the sleep stages. For a comatose patient, a test of the reactivity of the EEG signal to external stimulation is an important assessment tool for a clinician, since it provides significant information regarding the state and outcome of the patient. EEG reactivity may reveal potentially treatable conditions and also provide information of the level of drug-induced sedation. While some conclusions about the probability of a recovery can be drawn from the raw EEG signal as such, it has been shown that reactivity of the EEG signal to stimulation, i.e. a detectable change in the EEG signal after a stimulus as compared to the pre-stimulus situation, is a more specific indicator of a favorable outcome, cf. G. B. Young, et al: An Electroencephalographic Classification for Coma, Can. J. Neurol. Sci. 1997; 24: 320-325. Therefore, testing the EEG reactivity is an essential part of the EEG examination of a comatose patient. Moreover, the test of EEG reactivity provides information regarding the state of a patient for whom the GCS or another observational scoring system is not applicable. This is the case, for example, when neuromuscular blocking agents have been administered to the patient, which makes the patient unable to respond and thus the observational scoring systems inapplicable.
At present, the EEG reactivity is assessed by an EEG specialist trained to interpret EEG waveforms. In practice, ICU (Intense Care Unit) doctors or nurses, who are skilled in making GCS-type assessments, are usually not capable of interpreting the EEG waveforms, and therefore a consulting EEG specialist has to be called in for the test of EEG reactivity. Various types of stimuli, such as auditory (shouting the patient's name, blowing a horn) and somatosensory (pinching, squeezing, shaking) stimuli, may be applied in the test. The EEG specialist annotates the time instant of the stimulation and compares the recorded EEG signal before and after the annotated time instant. Often the EEG signal shows reactivity only to some of the given stimuli; in this case reactivity is considered to be present.
The test of the EEG reactivity is currently based mainly on manual stimulation performed by a nurse or a doctor, despite the fact that its intensity is often subject to variability between stimulators. The stimulation is typically given by manually touching the patient, i.e. the stimulation is sensed by the somatosensory system of the patient. Even if the same person repeats stimulation, its intensity may vary. Comparison of EEG reactivity between different stimulations is in this case difficult because a more intense stimulus may result in a higher response in the EEG signal than a milder stimulus. Standardized stimulation intensity would enable the evaluation of changes in patient's reactivity over a longer time period.
Standardized stimulation intensity can be achieved with an automatic stimulation device, such as an NMT (NeuroMuscular Transmission) module, which is used for evaluating muscle response by stimulating a peripheral nerve. However, the use of an automatic stimulation device capable of producing a standard stimulation pattern similar to the manual stimulation given by the nursing staff requires normally a dedicated actuator. This is a drawback especially in Intensive Care Units (ICU) and in operation rooms which tend to be crammed with medical appliances. Healthcare professionals are therefore reluctant to introduce new appliances to such points of care. This drawback may be alleviated to some degree by using a sensor arrangement in which all electrodes and sensors may be connected to a single connector, whereby the amount of cables or hoses between the patient and the monitoring devices may be reduced. A sensor arrangement like this is disclosed in U.S. Patent Application 2005/0085741 A1. The sensor arrangement comprises separate NMT electrodes to stimulate the facial nerves of the patient, and the NMT response is measured through recording electrodes or a mechanical sensor located in the facial area of the patient.
Another drawback related to manual stimulation is that the annotation of the time of the stimulation in the EEG signal may not always be accurate. A precise annotation of the stimulation instant in the EEG signal would facilitate more advanced signal analysis methods, for example averaging of responses to several stimuli.
Consequently, the test of EEG reactivity is complicated by the lack of a compact and automated stimulation arrangement that can provide constant stimulation patterns similar to or simulating the manual stimulation given by the nursing staff, and allows precise annotation of each stimulus with respect to the EEG waveform.
The present invention seeks to alleviate or eliminate the above-mentioned drawbacks.